A modern information storage system should acquire information quickly, maintain the information with integrity over time, and provide for the quick and accurate retrieval and erasing of the information. As the demand for miniature hand held communication and other data processing devices increases, the demand for smaller nonvolatile data storage devices increases. A flash memory device, such as erasable programmable read only memory (EPROM) or an electrically erasable programmable read only memory (EEPROM), is one type of information storage device used in modern nonvolatile information storage systems.
FIG. 1 is a cross-sectional view of a prior art flash memory cell 100. Flash memory cell 100 includes substrate 103, source region 106, drain region 109, channel 112, oxide 115, floating gate 118, dielectric layer 121, and control gate 124. Dielectric layer 121 comprises a three layer oxide-nitride-oxide dielectric.
In flash memory cell 100, information is stored on floating gate 118 as electronic charge. To facilitate the quick acquisition of information in flash memory cell 100, the cell is designed to have a large capacitance between control gate 124 and the floating gate 118. As the density of memory cells in a solid state memory is increased by scaling the physical dimensions of the cells, the area of floating gate 118 is decreased, which decreases the capacitance and coupling between control gate 124 and floating gate 118. Unfortunately, this decrease in coupling reduces the electric field between the substrate and the floating gate during a write operation. A decrease in the electric field decreases the rate at which hot electrons flowing in channel 112 are injected onto floating gate 118 and increases the time to store charge on the floating gate 118.
Information must be retained in flash memory cell 100 on floating gate 118 for a long period of time. As described above, as flash memory cells are scaled, the area of the floating gate in each cell is decreased and the capacitance between the floating gate and the control gate is decreased. A smaller capacitance results in less charge being injected into the floating gate. For a particular leakage current, a memory cell having less stored charge loses its information more quickly than a memory cell having more stored charge. One solution to this problem is to substitute a material having a high dielectric constant for dielectric 121 in flash memory cell 100. Unfortunately, materials that have a high dielectric constant and are compatible with integrated circuit manufacturing processes often have a high leakage current, which decreases the storage lifetime of the stored information for a fixed amount of charge.
Quick and accurate retrieval of information from flash memory cell 100 requires accurately sensing the charge stored on floating gate 118. A large amount of stored charge provides a signal that is easier to accurately sense than a small amount of stored charge. A large amount of stored charge also decreases the read time at the sensing device. Unfortunately, as devices are scaled to a small fraction of a micron, and the operating voltages are reduced, the charge on floating gate 118 is often decreased, thus making the quick and accurate retrieval of information more difficult. Flash memories already operate with relatively little stored charge, so as the stored charge is decreased further by scaling the physical dimensions of a memory cell, there is an increase in the number of errors at the sense amplifier when reading information from the scaled flash memory cell 100.
For these and other reasons there is a need for the present invention.